Electronic ballasts typically include an inverter that provides high frequency current for efficiently powering gas discharge lamps. Inverters are generally classified according to switching topology (e.g., half-bridge or push-pull) and the method used to control commutation of the inverter switches (e.g., driven or self-oscillating). In many types of electronic ballasts, the inverter provides a square wave output voltage. The square wave output voltage is processed by a resonant output circuit that provides high voltage for igniting the lamps and a magnitude-limited current for powering the lamps in a controlled manner.
When the lamps fail, or are removed, or begin to operate in an abnormal fashion, it is highly desirable that the inverter be shut down or shifted to a different mode of operation in order to protect the inverter and resonant output circuit from damage due to excessive voltage, current, and heat. Circuits that alter the operation of the inverter in response to lamp faults are usually referred to as inverter protection circuits.
In many existing ballasts that include inverter protection circuits, spurious electrical noise or a momentary variation in the lamp current, such as what may normally occur during the "break-in" period for a new fluorescent lamp, may be mistakenly interpreted as a lamp fault condition. Consequently, the inverter may be unnecessarily shut down or shifted to a different mode of operation. This poses a significant inconvenience to users and encourages wasteful replacement of functional lamps.
Additionally, many existing ballasts include no provision for ignition of lamps under low-temperature conditions at which the lamps may not properly ignite on the first attempt. In such ballasts, failure of the lamps to ignite on the first attempt is treated as a lamp fault condition. Several existing ballasts address this problem by employing "flasher" type protection circuits that periodically attempt to ignite the lamps. Flasher type circuits provide an indefinite number of ignition attempts and are therefore potentially useful for low-temperature starting. Unfortunately, flasher type protection circuits often produce sustained repetitive flashing in one or more lamps, a characteristic that has proven to be an annoyance to users/occupants.
Another problem common to many existing ballasts with inverter protection circuits relates to fault detection sensitivity. Ideally, a protection circuit should tolerate a certain amount of erratic behavior during lamp ignition without treating such behavior as a lamp fault condition, but should be considerably more sensitive during normal operation after the lamp has ignited. Many existing ballasts utilize the same lamp fault detection threshold during ignition and normal operation. In such circuits, in order to avoid false detection during lamp ignition, the fault detection threshold must be set somewhat high. Unfortunately, a high fault detection threshold has the unfavorable effect of precluding or interfering with the detection of legitimate lamp faults during normal operation, and may therefore limit the ability of the protection circuit to fulfill its intended purpose of preventing damage to the inverter and output circuit.
Many existing protection circuits require a large number of discrete components. This makes the ballast physically large, materially expensive, and difficult to manufacture. From the standpoint of reliability and manufacturability, it is highly desirable to have a ballast that requires only a modest amount of discrete lamp fault detection circuitry and that incorporates the greater portion of the protection logic and circuitry in an inverter control circuit that is well-suited for implementation as an integrated circuit.
It is therefore apparent that a need exists for an electronic ballast with an inverter control method and inverter control circuit that offers enhanced immunity to electrical noise and normal transient variations in lamp current, and that provides multiple ignition attempts for igniting lamps under low-temperature conditions, but that does not produce sustained flashing of the lamps. A need also exists for an electronic ballast with an inverter control method and inverter control circuit that includes an adjustable lamp fault detection threshold for decreased sensitivity during lamp starting and enhanced protection during lamp operation. A further need exists for an electronic ballast with an inverter control circuit that minimizes the required amount of discrete lamp fault detection circuitry and that is well-suited for implementation as a single integrated circuit. Such a ballast would offer improved operation, enhanced reliability, and greater ease of manufacture, and would therefore represent a significant improvement over the prior art.